I'm building my RCM 40 trainer. Smooth as silk building till the tank no biggie, the stopper pops in the tank good but it has a little groove in the middle of the stopper that sits in the tank opening. I can rotate the stopper with little force and isn't snug. Is this the way it is, will it expand when filled with fuel or do I have a problem?

pressure-test it with air under water and see if it leaks. If so; tighten the screw, add sealant, or buy another tank for like $7.

Does the screw compress the stopper to in a sense squash it and give it a wider diameter?

G'day Mate,
Yes it should be firm, so no fuel gets out & no air gets in, other than through the tubes.

I tightened up the screw and all is good. Thnx guys